A body sensible acoustic device is designed so that it transmits vibration from a driver unit to the body of the listener, i.e., to allow the person to feel sound through his or her body. In general, such a device is built into a chair or sofa so that it has a maximum contact area with the body. In the case when the device is built into a chair, both the device and the chair must perform their own functions satisfactorily.
An example of a conventional body sensible acoustic device built into a chair is shown in FIG. 1. In FIG. 1, reference numeral 1 designates the frames of the chair, which has a seat 1a and a back 1b; 2, driver units provided with sub-frames in the seat 1a and the back 1b, the driver units 2 being driven by low-frequency electrical input signals; and 3, canvas members spread on the frames 1 of the seat 1a and the back 1b, the canvas members 3 being made of a canvas material such as that used, for instance, for tents.
In this conventional body sensible acoustic body, low-frequency electrical signals are applied to the driver units 2 supported by the canvas members 3 so that vibration is transmitted to the human body. Cushions or the like can be placed on the, seat, 1a and the back 1b if necessary. In such a case, however, the vibration must be transmitted to the body through both the canvas member 3 and the cushions.
The conventional body sensible acoustic device thus constructed suffers from a drawback in that, when the driver units 2 vibrate, leakage sound is produced, heard as distorted sound, in the canvas members 3.
Regarding the canvas member 3 as a mechanically vibrating element, it is small in weight, large in stiffness, and small in resistance (or internal loss). Because of these characteristics, the lowest resonance frequency of the material is high, and therefore the vibration sensed by the human body is not sufficient in the low-frequency range.
The invention further relates to an electromechanical vibration converter which may be used as the driver in such a body-sensible acoustic device, and more particularly to an electromechanical vibration converter having increased ranges of the lowest resonance frequency and reduced resonance sharpness.
Examples of a conventional electromechanical vibration converter are a loudspeaker unit and a transducer in a body-sensible acoustic device. Typical examples are shown in FIGS. 2 and 3. In the converter, a magnetic circuit 42 is provided in an outer case 41, and an annular damper 43 (FIG. 3) is constructed in such a manner that its inner peripheral portion is held between a top plate 42A and a magnet 42B and its outer peripheral portion is secured to the outer case 41. A cap 41A is engaged with the outer case 41 in such a manner that the outer peripheral portion of the damper 43 is pushed against the outer case 41 by the cap 41A.
One end of coil bobbin 44 is fixedly secured to the cap 41A. A drive coil 44A wound on the coil bobbin 44 is positioned in the air gap of the magnetic circuit 42.
When a drive current is supplied to the drive coil 44A, a drive force is produced in the coil bobbin 44 by the magnetic flux formed in the air gap, thus moving the coil bobbin 44. In this operation, the magnetic circuit 42, being elastically suspended by the damper 43, is moved relative to the outer case 41. Therefore, if the outer case 41 is secured to an element to be vibrated which has an appropriate mechanical impedance, then the converter can vibrate according to the output signal of the element.
As is apparent from the above description, the damper 43 is generally made of a thin plate, and therefore it cannot provide a sufficiently high internal loss. Accordingly, the converter has an extremely high resonance sharpness, and consequently a narrow effective frequency band for reproduction. That is, the converter has a poor transient response.
Furthermore, in this converter, the tension of the damper 43 is utilized to suppress the excessively large amplitude due to the high resonance sharpness. As a result, when the input is high, the damper responds to it as indicated by the waveform shown in FIG. 4, that is, with the peaks collapsed. Therefore, if the converter is used for a long period of time, the damper has a tendency to break.
The center of gravity of the magnetic circuit 42 does not coincide with the position of support of the damper 43. Not only because of this fact, but also because of the size of the magnetic circuit, a rolling phenomenon is liable to occur, thus causing irregular vibration or residual vibration.